1. Technical Field
The present invention relates to a method for generating a dither mask that is used in a halftone process in a printing device.
2. Related Art
An ink jet line printer performs printing while transporting a printing sheet by ejecting ink onto the printing sheet from nozzles of printing heads that are arranged across the entire width of the printing sheet in a direction crossing the direction in which the printing sheet is transported. The printer typically includes a plurality of the printing heads. In addition, the printing heads are arranged so that regions (hereinafter also referred to as overlapping regions) of adjacent printing heads overlap each other in a direction in which the printing heads are arranged. The adjacent printing heads eject ink from the overlapping regions and form an image on the printing sheet.
An irregularity in the density of the image or a reduction in granularity may occur in a portion of the image formed by the line printer in a region that corresponds to the overlapping regions. These problems occur due to the fact that: ink is ejected from the nozzles of the printing heads; dots are formed on the printing sheet and shifted from target positions; and an irregularity in the density of dots is locally present due to the shifting of the dots. The shifting of the ink dots from the target positions may caused by the fact that the attached printing heads are misaligned from appropriate positions due to a problem with the attachment accuracy or by the fact that a distance that the printing sheet is transported is not exactly constant due to a problem with mechanical accuracy.
Those problems not only occur in line printers but also may occur in a band-pass scanning type serial ink jet printer that completes a raster by performing main scanning mainly once and has overlapping portions that complete a raster by partially performing main scanning twice. This is due to the fact that the aforementioned problem with the accuracy of transporting a printing sheet also occurs in the serial ink jet printer.